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Under achievers The notion of the clever crow has been delivered a blow with a New Zealand study showing the birds do not mentally plan how to "solve problems".

Dr Alex Taylor, at the University of Auckland, says New Caledonian crows are known to be among the fastest in the world at solving certain tasks spontaneously.

One of the most common examples is a string-pulling task that involves hanging food from a piece of string.

Taylor says New Caledonian crows can, without a single mistake, pull up the string to obtain the food. This action requires them to pull up a segment of string and then step on it to prevent it from dropping.

However despite string pulling having been documented as much as 400 years ago, researchers still do not understand what cognitive processes are used by the birds to solve the problem, says Taylor.

"One theory is that crows are coming up with some sort of plan and executing the plan," says Taylor, of the Department of Psychology.

In a study published in the latest Proceedings of Royal Society B, Taylor discounts this theory by showing when the stimulus of food moving towards them is removed, crows fail a simple string-pulling task.

He says this shows that rather than having an "insight" into how their actions will help them gain food, the string pulling is motivated by the sight of food getting closer.

Coiled ropes

For the latest study Taylor and his team took 11 wild crows and after habituating them to rope, presented them with two pieces of coiled rope, both of which appeared to have meat attached to the end.

However one coil was continuous, while the other consisted of two segments with a 10-centimetre gap between the two pieces.

Taylor says because the ropes were coiled this meant when the crows pulled on the ropes, the meat did not immediately move closer, thereby removing the stimulus of a reward for their actions.

He says the experiment was also testing whether the crows understood string connectivity and would choose to pull the continuous coil over the discontinuous one.

However Taylor says individual crows showed no preference for the continuous coil and only one crow completed the experiment.

All other crows failed to pull the continuous string a sufficient number of times for the meat end of the rope to begin moving.

"If the crows had been mentally simulating their interaction with the string, they should have been able to predict the effect of repeated pulling and so been motivated to carry on string pulling without feedback [of a reward]," Taylor and colleagues write.

"Our results clearly demonstrate that such motivation was lacking."

Validation

To ensure the validity of the experiment, Taylor and his team also trialled the experiment with 50 students to ensure they would understand that pulling the end of the connected rope would lead to a reward. He says 49 of the students did this part of the experiment successfully.

Taylor says previous studies that show New Caledonian crows can solve vertical-string-pulling problems and the results of his study suggest crows can spontaneously solve problems without planning their actions.

"Crows aren't clever, they are only 'clever' at certain aspects of their life," says Taylor.

He says the fact not all birds are proficient at string pulling suggests there is some "particular brain hardware" also involved in the crows' actions.

"While spontaneous string pulling in birds does not require complex cognitive software, it may well require specific neural hardware," the researchers write.

Taylor says this work and previous studies with the New Caledonian crow suggest researchers need to rethink how to test intelligence in animals.

"What all these papers are showing is more careful thought needs to be given to how to test these abilities and more focus should be on highlighting when these things don't work out … [which is] equally important in understanding the evolution of intelligence."